Current approaches to cancer treatment suffer from a lack of specificity. The majority of drugs that have been developed are natural products or derivatives that either block enzyme pathways or randomly interact with DNA. Due to low therapeutic indices, most cancer treatment drugs are accompanied by serious dose-limiting toxicities. The administration of drugs to treat cancer kills not only cancer cells but also normal non-cancerous cells. Because of these deleterious effects, treatments that are more specific for cancerous cells are needed.
It has been found that a class of genes, the oncogenes, plays a large role in the transformation and maintenance of the cancerous state and that turning off these genes, or otherwise inhibiting their effects, can return a cell to a normal phenotype. The role of oncogenes in the etiology of many human cancers has been reviewed in Bishop, “Cellular Oncogenes and Retroviruses,” Science, 235:305–311 (1987). In many types of human tumors, including lymphomas and leukemias, the human bcl-2 gene is overexpressed, and may be associated with tumorigenicity (Tsujimoto et al. Involvement of the bcl-2 gene in human follicular lymphoma, Science 228:1440–1443 (1985)).
Antisense oligodeoxynucleotides are one example of a specific therapeutic tool with the potential for ablating oncogene function. These short (usually about 30 bases) single-stranded synthetic DNAs have a complementary base sequence to the target mRNA and form a hybrid duplex by hydrogen bonded base pairing. This hybridization can be expected to prevent expression of the target mRNA code into its protein product and thus preclude subsequent effects of the protein product. Because the mRNA sequence expressed by the gene is termed the sense sequence, the complementary sequence is termed the antisense sequence. Under some circumstances, inhibition of mRNA would be more efficient than inhibition of an enzyme's active site, since one mRNA molecule gives rise to multiple protein copies.
Synthetic oligodeoxynucleotides complementary to (antisense) mRNA of the c-myc oncogene have been used to specifically inhibit production of c-myc protein, thus arresting the growth of human leukemic cells in vitro, Holt et al., Mol. Cell Biol. 8:963–973 (1988), and Wickstrom et al., Proc. Natl. Acad. Sci. USA, 85:1028-1-32 (1988). Oligodeoxynucleotides have also been employed as specific inhibitors of retroviruses, including the human immunodeficiency virus (HIV-I), Zamecnik and Stephenson, Proc. Natl. Acad. Sci. USA, 75:280–284 (1978) and Zamecnik et al., Proc. Natl. Acad. Sci. USA, 83:4143–4146 (1986).